1. Field of the Invention
The present invention is directed to a novel activator means for use in activating or re-activating adhesive and sealant compositions that have been pre-applied to a bonding surface or a substrate prior to mating said bonding surface with the surface or substrate to which it is to be bonded. In one embodiment, the activator means is a heated activator means for use with pre-applied heat activated/re-activated adhesive and sealant compositions wherein the heat of the activator means transforms the adhesive or sealant into a flowable state and the features of the activator face of the activator means serve to lift the flowable adhesive or sealant from the substrate surface, collect and then redeposit the same on the substrate surface in the area of the intended bondline in a defined profile. In a second embodiment, the activator means has an activator that is not heated or, if heated, provides minimal heat, and whose activator face has features that are capable of lifting the adhesive from the surface of the substrate to which it is pre-applied, mixing and collecting the pre-applied adhesive material and then re-depositing the same on the substrate surface in the area of the intended bondline. Preferably, the activator means is employed with pre-applied, two-part, curable or polymnerizable adhesive or sealant compositions, especially encapsulated compositions, and has features on the activator face that are capable of intimately mixing the two parts to activate the cure or polymerization of the composition before re-depositing the activated composition on the substrate surface in the area of the intended bondline. The present invention is also directed to industrial bonding systems and apparatus that incorporate said activator means, especially systems and apparatus for high-speed industrial, automated bonding applications as well as bonding methods employing the aforementioned activator means.
2. Description of Related Art
Multi-part adhesive and sealant compositions are widely known and commercially available for both consumer and industrial or commercial use. These compositions are typically characterized as being reactive systems wherein cure or polymerization is initiated once all of the components of the curable composition are brought into intimate contact with one another. By far, the most common multi-part adhesive and sealant compositions are two-part systems. Two-part adhesive and sealant compositions take many forms and can be employed in numerous ways. For example, an adhesive or sealant composition may comprise a first part, Part A, which includes a liquid curable component, and a second part, Part B, which includes an activator in an appropriate solvent, which activator, in the presence of Part A, initiates or effectuates cure or polymerization of the liquid curable component. In use, one or both of the surfaces to be bonded is pretreated or primed with Part B so that when Part A is applied thereto or to the untreated surface, if one, and the surfaces mated, cure or polymerization is initiated.
Alternatively, both Parts A and B may comprise the same or a different liquid curable component with Part A also containing an activator which is, by itself, non-reactive with the liquid curable component of that part, but which is reactive when mixed with Part B. In this instance the activator may be a chemical additive such as an initiator, accelerator, catalyst, etc., which effectuates polymerization or cure but does not itself become chemically incorporated into the cured or polymerized polymer (though it may be physically entrained therein) or it may be a component co-polymerizable with the liquid curable component of Part B, as for example a cross-linking agent or hardener. Of course, not all two-part systems are liquid curable or polymerizable systems. For example, two-part epoxy adhesive and sealant compositions often are found in the form of ribbons, strings, beads, etc. of the two parts in a putty-like state, extruded in a side-by-side or concentric relationship. As with liquid curable or polymerizable systems, cure or hardening is initiated upon intimate mixing of the two components.
In industrial operations, most two-part adhesive and sealant compositions are applied at the time of use: typically through the use of expensive, sophisticated dispensing equipment which meters the relative proportions of the two components and mixes the two concurrent with or just prior to dispensing. Whether mixing occurs within the dispensing equipment or upon exiting the same depends upon the physical nature of the materials as well as their cure characteristics, especially the speed of cure or pot life of the activated materials. Such dispensing equipment is often fed from large containers or reservoirs of the two components or the dispensing equipment may be constructed so as to receive disposable cartridges, including cartridges comprising two chambers, e.g., double barrel cartridges, each of which is constructed to contain a predetermined amount of the two components so that the two materials are dispensed in their proper proportions. The former allows for continuous, or nearly so, dispensing so long as the reservoirs are replenished. However, concerns arise relative to clogging of the dispenser nozzle due to cure of the two-part compositions in the mixer head or at the nozzle orifice, particularly in the event of a shut down or temporary stoppage of the assembly line. The use of the cartridge is especially beneficial in those systems where it is impractical to employ large containers or the containers are unable to be located proximate to the dispenser nozzle. Further, with these systems one has the benefit of being able to dispose of the spent cartridges and avoid concerns of contamination, set-up or cure of the large reservoirs. Furthermore, if a problem arises the unspent cartridge is merely replaced with a new cartridge; whereas, if a problem arises with the large reservoir systems, the reservoirs must be emptied and cleaned and the dispensing system purged before dispensing can be resumed. With cartridges, one also has the possibility of integrating the nozzle or mixer head in the cartridge assembly. Thus, the nozzle or mixer head is replaced with each replacement of the cartridge. If a problem should arise where the nozzle or mixer head is clogged, one merely replaces the whole cartridge system and gets on with the assembly operation. The only loss is the unspent materials in the cartridge and a minimal delay in the assembly line process. Despite the attributes of the cartridge systems, with or without the integrated nozzle or mixer head, the cartridges must be continually replaced due to the relatively low volume of materials held within them as compared to the larger reservoirs of the more sophisticated dispensing equipment. This replacement process results in a repetitive, though temporary, stoppage of the assembly line with its concomitant impact on through put.
Perhaps the greatest disadvantage of the foregoing systems is the use and application of liquid materials in high-speed industrial manufacturing lines. Extreme care must be employed to ensure that the components are mixed in their proper proportions. Even small variations in the proportions by which the two components are dispensed may lead to materials that cure too quickly or too slowly, if at all. Even if cure is unaffected by such variations, such variations will likely result in adhesives having different performance characteristics or physical profiles than would be attained had the dispensing occurred as intended. Furthermore, because these dispensing systems rely upon pressure, direct or indirect, to dispense the components, any variation in pressure may also adversely affect the adhesive and/or bond to be formed. For example, low pressure or the loss of pressure will result in the dispensing of insufficient quantities of adhesive or gaps in the dispensed adhesive leading to poor or failed bonds. Too much pressure or sudden surges in pressure will lead to the dispensing of excessive quantities of material rendering the substrates unsuitable for use or, worse, broadcasting adhesive material not only onto the substrate but onto proximate components of the assembly line itself as well. The latter may lead to a shut down of the assembly line in order to clean it before more, and oftentimes more significant, problems arise.
In order to avoid many of the problems associated with in-line applied liquid adhesive materials, many industrial processes have employed pre-applied adhesives. The vast majority of these systems employ thin films of a dry-to-the-touch thermoplastic (including cooled hot melt) or thermoset adhesive materials that are activated or re-activated, as the case may be, upon exposure to, among others, high temperatures, irradiation (e.g., UV, IR, etc.), electromagnetic fields (e.g., RF, UHF, microwave, etc.) and/or ultrasound. Though such systems have found broad use, they too have limitations, particularly with respect to the bonding of surfaces which are not planar or where gaps may exist. Furthermore, each presents concerns relative to the generation of the necessary energy to activate or re-activate the adhesive, not only with respect to the substrate being acted upon but also the associated equipment proximate to the activation station of the assembly line as well as other general health, safety and environmental concerns, particularly with respect to workers responsible for the operation and maintenance of the assembly line. For example, high-speed packaging formation and closing processes typically employ a pre-applied thermoplastic or thermoset adhesive film applied to the bonding surface which is heated to its activation or melt temperature by one or more heater elements that direct a jet of extremely hot air to the intended bondline.
Although such jets of hot air can be directed, the greater the distance from the source to the substrate, the more diffuse that jet becomes. Also, once the jet hits the substrate, the hot air then disperses along the surface of the substrate. This can be a significant problem with many substrates that have heat sensitive surfaces, coatings and the like. For example, packaging oftentimes has a coating or varnish and/or printing on its outer surface which may be adversely affected by the high temperatures needed to activate or reactive the pre-applied adhesive material, particularly if the heat cannot be contained to the immediate bondline area. Furthermore, workers who are called upon to address problems with the assembly line often come close to or in direct contact with the heating equipment or the heat generated by the equipment, particularly the jets of hot air, which heat may cause severe burns. While safeguards and protective means or design modifications may be incorporated into the assembly line and the heating equipment, such requires additional expenditure and adds more complexity to the system. The same holds true for systems that rely upon electromagnetic energies, ultraviolet light, etc.; rather than the direct application of heat. Finally, such systems are often lengthy, adding further cost, both in terms of equipment and space, to the process in order to allow sufficient cooling of the heated adhesive or sealant before the bond is formed or before sufficient bonding is achieved so that the substrate can move on to its next workstation.
Another class of pre-applied materials are those known as the encapsulated adhesives and sealants. Such encapsulated systems may comprise a one-part adhesive, such as the encapsulated PSAs of Schwantes et. al. (U.S. Pat. No. 6,592,990) or the encapsulated tacky adhesives of Eichel (U.S. Pat. No. 2,986,477), or two- or more part adhesive systems, such as solvent based encapsulated adhesive systems as in Roesch et. al. (U.S. Pat. No. 5,922,798) and Eichel (U.S. Pat. No. 2,907,682) or, more commonly, the liquid curable systems as in Bachmann et. al. (U.S. Pat. No. 3,814,156 and U.S. Pat. No. 3,826,756), Chao (U.S. Pat. No. 6,375,872) and Usami et. al. (U.S. Pat. No. 5,397,812). With the former, crushing the microcapsules makes the encapsulated adhesive available for effectuating a bond. In the latter, crushing the microcapsules merely creates the opportunity for the adhesive to activate or be formed. For example, in the solvent based systems the microcapsules may contain, or one type may contain, a solvent that dissolves or renders tacky an adhesive material that exists in a separate microcapsule, as a particle or as the binder that holds the microcapsule to the surface. When the solvent microcapsule is crushed, the solvent is released so as to ‘activate’ the adhesive. Alternatively, with the curable systems, cure cannot be initiated or activated until the necessary curatives, whether they be activators, catalysts, initiators or whatever, come in contact with each other and/or the polymerizable components. Crushing the microcapsules allows for all such constituents to come into contact so that cure can be effectuated and the adhesive or sealant formed in-situ.
As mentioned, the more common of the encapsulated adhesives are the two-part liquid curable systems. Like the in-line applied liquid curable systems mentioned previously, these encapsulated adhesives can be custom formulated to fit any number of particular applications/end-uses and/or achieve a multitude of performance properties. Yet, despite their broad properties and performance profiles, these adhesive have found very limited commercial applications. By far, the most common use of these materials is in mechanical fastening applications, either threaded assemblies or in retaining applications. In the former, activation is achieved by the threading action. In the latter, activation is achieved as a result of the assembly of the two parts to be bonded and is most often associated with insertion assemblies. For example, Bonutti (U.S. Pat. No. 4,750,457) employs an encapsulated adhesive in the grooves set in the sidewall of an engine cup plug that is to be inserted into the receiving hole of an engine block. An interference fit deforms the grooves leading to the fracturing of the microcapsules and a flow of the liquid components contained therein. Similarly, Müller et. al, (U.S. Pat. No. 4,100,954) and Mederski (U.S. Pat. No. 5,965,866) employ encapsulated adhesives in the recesses of a body or card wherein the force of inserting a dowel or microchip, respectively, into the recess crushes the microcapsules, thereby initiating cure of the adhesive material.
The use of encapsulated adhesives has also been found with the bonding of rigid flat or planar substrates, albeit in very limited applications. Since activation is initiated by crushing of the microcapsules and the microcapsules themselves are of extremely small diameters, their use, in this regard, is limited to substrates whose bonding surfaces are planar or, if contoured, whose contours mirror image one another so that the two mate together with minimal gap at the bondline or bond interface. If gaps exist which are greater than the diameter of the microcapsules or, depending upon the fragility or rigidity of the capsule walls, even greater than three quarters the diameter of the microcapsules, there may be insufficient release of the curable components so that no or poor bonding occurs. On the other hand, if the mating is perfect and no gap exists whatsoever, then there is concern that all or substantially all of the liquid adhesive material will be squeezed out under the pressure used to crush the microcapsules. Here, remnants of the microcapsule walls help maintain a gap between the mated surfaces to preclude all the adhesive material from being squeezed out under the mating pressure. Even so, a substantial amount of adhesive material may be pressed out of the bondline resulting in poor bond strengths as well as excess adhesive that must be cleaned up.
A growing area of use for encapsulated adhesives is in the bonding of prous or semi-porous, flexible sheet media, especially paper and paper products as well as woven and non-woven fabrics, sheets and web type products. For example, Akridge et. al. (U.S. Pat. No. 5,794,409) and Haugwitz (U.S. Pat. No. 4,961,811) employ encapsulated liquid curable adhesives in the production and/or closure of paper envelopes for letters, junk mail and the like. More recently, Schwantes et. al. (U.S. Pat. No. 6,592,990), employ encapsulated, in-situ formed PSAs for label bonding applications. Activation of the adhesive is typically accomplished by passing the mated surfaces through one or more pinch rollers, under one or more stationary blades, past a set of rotatable discs or a series of sets of rotatable discs, or a combination of the foregoing (See, e.g., Wells et. al., U.S. Pat. No. 6,726,796), which operation fractures the microcapsules and spreads the adhesive between the two mated substrates. Alternatively, at least with the in-situ formed PSAs, the aforementioned activation means may act directly upon the pre-applied adhesive prior to mating the substrates to be bonded.
Despite the success of such systems, these too have limitations, particularly with respect to the encapsulated liquid curable adhesives. For example, due to their low viscosity, liquid curable compositions have a tendency to wick into the porous substrates leaving little curable material in the bond gap or interface to create the bond. This is not as significant a problem with thinner media and very planar surfaces as found with conventional paper envelopes where the liquid materials often saturate the immediate surface layer of the paper, which saturation provides sufficient adhesive material to effectuate the bond. However, more significant concerns and problems are found with thicker media such as paperboard and especially cardboard, where the liquids can be absorbed or wicked deep into the subsurface, leaving little, if any, liquid curable material at the interface, and certainly an insufficient amount to address surface irregularities often found with these materials. On the other hand, some degree of porosity or surface irregularity may be important for those substrates wherein the pre-applied adhesive is to be activated by the use of the aforementioned pinch rollers, blades and rotatable discs; otherwise, if such pores were not present to serve as receptacles for the adhesive, such means would have a tendency to push away, squeeze out or scrape off the adhesive materials leaving little at the bond line or bond interface.
Another factor limiting the use of encapsulated adhesives is the inability to supply sufficient pressure to rupture the microcapsules and/or the poor efficiency with which the capsules are ruptured. Exacerbating the problem is the use of thicker capsule walls as is often found with traditional encapsulated adhesives and sealants in order to avoid premature fracturing. These problems are especially problematic with substrates like paper, cardboard, and the like, especially in thicker sections, that absorb or cushion the pressure that may otherwise be exerted by activation means as described above. Making adjustments to increase the pressure will have the deleterious effect of marring or otherwise deforming the substrates. Substrates that are rigid have a similar problem but for a different reason. Specifically, while the pressure exerted on the rigid substrate passes through to the bond interface, if the substrate surfaces are irregular, there may be gaps at the bond interface where no matter how much pressure is supplied, no fracturing of the microcapsules in the gaps occurs. Thus, again, poor bonding or areas of no bonding may exist.
Though some of the aforementioned problems and concerns are lessened in systems like Wells et. al. wherein a mechanical activator means is seen acting directly upon the encapsulated adhesive materials, such a configuration or process is not typical of nor applicable to the majority of pre-applied encapsulated adhesives. Like conventional activation apparatus and methods, Wells et. al, rely upon compressive forces to fracture the microcapsules; however, since Wells et. al, act directly upon the adhesive, rather than the mated substrates, additional concerns arise and accommodations need to be made to avoid scraping or squeezing all the activated adhesive or sealant off the substrate surface. Here, the angle of the activator means to the substrate surface is very low to enable a mashing of the pre-applied adhesive or sealant as well as allow for the activated adhesive or sealant to pass under the activator means. In any event, the apparatus of Wells et. al. is further limited to those applications which can accommodate or only need a thin film of adhesive for effectuating a bond.
Despite the many attributes of encapsulated curable adhesive systems, particularly with respect to the ability to remove the dispensing of liquid and/or molten adhesives from assembly lines, current technology has not advanced to the point where such may be used across a broad spectrum of substrates and applications. For example, limitations on microencapsulation technology, particularly with respect to the difficulty in, if not inability to, microencapsulate highly viscous materials, effectively restricts the creation of encapsulated adhesives and sealants to those of low viscosity liquid materials. Regardless, perhaps the key shortfall in such technology has been with respect to the activation of encapsulated adhesives. As noted above, most commercial applications rely upon the mating of the substrates to effectuate the bond, either through the rotation or threading action of threaded assemblies or the forced insertion of one element into another in the retaining applications. While rollers, blades and/or rotating discs, as mentioned in Wells et. al. above, facilitate the use of such encapsulated adhesives on certain substrates, their use on rigid and/or non-porous substrates is suspect. Furthermore, because such devices provide only a thin film of adhesive material, they do not allow for their use on non-planar substrates or substrates where gaps may exist, either due to design, poor quality control, or natural flex forces in the materials from which they are made.
Thus, it would be desirable to and there is a need in the industry for a means for activating or re-activating a pre-applied, heat activated thermoset or thermoplastic adhesive or sealant composition which means directly transfers heat to the adhesive or sealant composition without concern for heating, particularly to adversely high temperatures, areas of the bonding substrate outside of the bonding area and which means has minimal heated surface area so as to minimize the possibility that workers in the area or working on the assembly line will be burned or exposed to the high temperatures.
In another respect, it would be desirable to and there is a need in the industry for a means for activating a pre-applied adhesive material that does not require the use of heat, irradiation, ultrasound, electromagnetic energy, etc., for effecting activation or re-activation of the adhesive composition and where, if heat were employed, such heating is merely ancillary to the activation or re-activation of the adhesive and of relatively low temperature and limited duration.
Further, it would be desirable and there is a need in the industry for a means for activating a two-part pre-applied curable or polymerizable adhesive or sealant composition as well as for effectively and efficiently releasing an encapsulated adhesive or sealant composition, which means is simple to employ and cost effective and which is able to do so in other than as a thin film. In particular, it would be desirable and there is a need in the industry for a means for effectively activating a two-part, pre-applied adhesive or sealant composition, most especially an encapsulated liquid curable adhesive or sealant composition, that provides for excellent intermixing of the components thereof, especially those that are of higher viscosity and/or are non-flowing under ambient conditions, as well as a high degree of capsule fracture, where appropriate, while also leaving the activated adhesive on the substrate in a given profile or bead, especially one of a thickness that is of the same thickness as or thicker than the original pre-applied material. In following, it would be desirable and there is a need for such a means that also has the ability to deposit the activated adhesive in a customized bead of a given thickness for the particular end-use application to which it is applied.
Finally, it would be desirable and there is a need in the industry for a bonding method and apparatus which avoids the need to dispense in-line liquid curable or molten adhesive or sealant materials and which provides for high speed bonding applications with minimal capital investment, particularly as compared to current systems for activating pre-applied adhesive and sealant compositions. Furthermore, it would be desirable and there is a need for a high-speed industrial bonding method and apparatus that is able to employ a pre-applied adhesive or sealant composition, particularly one of a thin profile or thickness, and create, in-situ, an activated bead of adhesive or sealant material of a defined profile and/or thickness so as to accommodate the bonding of substrates that do not have planar or mating surfaces, especially those wherein the bonding areas have gaps and/or the whole of the bondline has a gap.